1. Field of the Invention
The disclosure relates generally to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bit assemblies. Still more particularly, the disclosure relates to percussion hammer drill bit assemblies including a driver sub with a guide sleeve portion.
2. Background of Related Art
In percussion or hammer drilling operations, a drill bit mounted to the lower end of a drillstring simultaneously rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target formation. The borehole created will have a diameter generally equal to the diameter or “gage” of the drill bit.
Referring to FIGS. 1-3, a conventional percussion drilling assembly 10 for drilling through formations of rock to form a borehole is shown. Assembly 10 is connected to the lower end of a drillstring 11 (FIG. 3) and extends along a central longitudinal axis 15. Assembly 10 includes a top sub 20, a driver sub 40, a tubular case 30 axially disposed between top sub 20 and driver sub 40, a piston 35 slidably disposed in the tubular case 30, and a hammer bit 60 slidingly received by driver sub 40. A feed tube 50 extends between top sub 20 and piston 35.
The upper end of top sub 20 is threadingly coupled to the lower end of drillstring 11 (FIG. 3), and the lower end of top sub 20 is threadingly coupled to the upper end of case 30. Further, the lower end of case 30 is threadingly coupled to the upper end of driver sub 40. As previously described, hammer bit 60 is slidingly disposed within driver sub 40. In particular, a series of axial mating splines 61, 41 on bit 60 and driver sub 40, respectively, allow bit 60 to move axially relative to driver sub 40 while simultaneously allowing driver sub 40 to rotate bit 60 along with drillstring 11 and case 30.
Hammer bit 60 is generally cylindrical in shape and includes a radially outer skirt surface 62 aligned with or slightly recessed from the borehole sidewall and a bottomhole facing bit face 64. The earth disintegrating action of the hammer bit 60 is enhanced by providing a plurality of cutting elements (not shown) that extend from the cutting face 64 for engaging and breaking up the formation. The cutting elements are typically inserts formed of a superhard or ultrahard material, such as polycrystalline diamond (PCD) coated tungsten carbide and sintered tungsten carbide, that are press fit into undersized apertures in bit face.
A guide sleeve 32 and a bit retainer ring 34 are disposed in case 30 axially above driver sub 40. The upper end of guide sleeve 32 slidingly receives the lower end of piston 35 and the lower end of guide sleeve 32 slidingly receives the upper end of hammer bit 60. Bit retainer ring 34 is disposed about the upper portion of hammer bit 60 axially between driver sub 40 and guide sleeve 32. Bit retainer ring 34 extends radially into an annular recess in the outer surface of hammer bit 60 proximal its upper end, and prevents hammer bit 60 from falling out of and completely disengaging driver sub 40.
A retainer sleeve 70 is coupled to driver sub 40 and extends axially downward from driver sub 40 along the outer periphery of hammer bit 60. Retainer sleeve 70 generally provides a secondary catch mechanism that allows the lower enlarged head of hammer bit 60 to be extracted from the wellbore upon lifting of the drillstring 11 and percussion drilling assembly 10 in the event of a crack or break in the shank (rotational drive) section of bit 60.
During drilling operations, a compressed fluid (e.g., compressed air, compressed nitrogen, etc.) is delivered down the drillstring 11 from the surface to percussion drilling assembly 10. In most cases, the compressed fluid is provided by one or more compressors at the surface. The compressed fluid serves to axially actuate piston 35 within case 30. As piston 35 moves reciprocally within case 30, it cyclically impacts hammer bit 60, which in turn cyclically impacts the formation to gouge, crush, and break the formation with the cutting elements mounted thereon. The compressed fluid ultimately exits the bit face 64 and serves to flush cuttings away from the bit face 64 to the surface through the annulus between the drillstring and the borehole sidewall.
During drilling operations, drillstring 11 and drilling assembly 10 are rotated. Mating splines 41, 61 on driver sub 40 and bit 60, respectively, allow bit 60 to move axially relative to driver sub 40 while simultaneously allowing driver sub 40 to rotate bit 60 with drillstring 11. As a result, the drillstring rotation is transferred to the hammer bit 60. Rotary motion of the drillstring 11 may be powered by a rotary table typically mounted on the rig platform or top drive head mounted on the derrick. The rotation of hammer bit 60 allows the cutting elements of bit 60 to be “indexed” to fresh rock formations during each impact of bit 60, thereby improving the efficiency of the drilling operation. Without indexing, the cutting structure extending from the lower face 64 of the hammer bit 60 may have a tendency to undesirably impact the same portion of the earth as the previous impact. Experience has demonstrated that for an eight inch hammer bit (e.g., hammer bit 60), a rotational speed of approximately 20 RPM (revolutions per minute) and an impact frequency of approximately 1600 BPM (beats per minute) typically result in relatively efficient drilling operations. This rotational speed translates to an angular displacement of approximately 5 to 10 degrees per impact of the bit against the rock formation.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole section by section. Once the drillstring has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drillstring, which again must be constructed section-by-section. As is thus obvious, this process, known as a “trip” of the drillstring, requires considerable time, effort and expense.
As previously described, in most conventional hammer bit assemblies, the driver sub (e.g., driver sub 40) and the guide sleeve (e.g., guide sleeve 32) are manufactured and installed as separate and distinct components that are axially spaced apart by the retainer ring (e.g., retainer ring 34). The driver sub and guide sleeve are typically designed and manufactured to include dimensional tolerances sufficient to allow for some movement, both axial and radial movement, within the percussion drilling assembly (e.g., assembly 10). During drilling operations, the repeated impacts and vibrations often causes the guide sleeve and the driver sub to move axially within the assembly. Such movements may result in undesirable surface wear of the driver sub and the guide sleeve, thereby increasing the tolerances and spacing with neighboring components and further exacerbating the movement and associated wear of the driver sub and the guide sleeve. Thus, over extended drilling operations, the relative movement and vibration of the guide sleeve and the driver sub often results in the undesirable and detrimental wear to the surfaces of the driver sub and the guide sleeve, thereby increasing the tolerances and gaps between the guide sleeve, the driver sub, and the surrounding components of the assembly. These increased tolerances allow for increased relative movement and associated wear, thereby promoting a vicious cycle that may potentially lead to breakage and/or damage to the driver sub, the bit retainer rings, the guide sleeve, or combinations thereof. Once the driver sub or guide sleeve is damaged, the entire drillstring (e.g., drillstring 11) must be pulled to replace the damaged component(s). Moreover, if the wear between the mating components is substantial, the timing of the hammer may be adversely affected, thereby reducing drilling efficiency.
Accordingly, there is a need for devices and methods that enhance the durability of a percussion drilling assembly. Such devices and methods would be particularly well received if they were relatively inexpensive, simple to manufacture, and did not otherwise interfere with the operation of the percussion drilling assembly.